1. Field of the Invention
The present invention relates to a semiconductor device comprising a plurality of thin film transistors formed on a substrate having an insulating surface, and a method for fabricating the same. More particularly, the present invention relates to a semiconductor device using thin film transistors having a crystalline Si film as an active region, and a method for fabricating the same.
2. Description of the Related Art
Recently, attempts have been made to form a high performance semiconductor element (for example, thin film transistor (TFT)) on an insulating substrate such as glass, or an insulating film for the development of a large-scale high-resolution liquid crystal display device, a low-cost monolithic-type liquid crystal display device comprising a driver circuit formed on the substrate where TFTs are formed, a high-speed, high-resolution adherent-type image sensor, a three-dimensional IC, and the like. Generally, a thin film silicon (Si) semiconductor is used for the semiconductor elements of these devices.
The thin film Si semiconductor is roughly classified into an amorphous Si semiconductor (a-Si) and a crystalline Si semiconductor.
It is possible to readily prepare an amorphous Si semiconductor by a vapor phase method due to its lower preparation temperature. Therefore, since an a-Si semiconductor has excellent productivity, the a-Si semiconductor has been generally used in the art. However, an amorphous Si semiconductor has a drawback in that it has poorer electric characteristics such as conductivity than a crystalline Si semiconductor. For this reason, it is difficult to apply an amorphous Si semiconductor to a semiconductor device that requires higher speed characteristics. Therefore, a semiconductor device comprising a crystalline Si semiconductor has been strongly demanded for the development of a semiconductor device having higher speed characteristics. The crystalline Si semiconductor includes polycrystalline Si, microcrystalline Si, and amorphous Si containing a crystalline component, and the like.
A known method for obtaining such crystalline thin film Si semiconductors includes the following three methods: (1) a method directly forming a crystalline film; (2) a method including forming an amorphous semiconductor film, and crystallizing the amorphous semiconductor film by applying thermal energy; and (3) a method including forming an amorphous semiconductor film, and crystallizing the amorphous semiconductor film with laser beam (laser light) energy.
In the above-described method (1), it is difficult to obtain a crystalline Si having a large grain size, since the crystallization proceeds simultaneously with the film formation. Thus, in this method, the thickness of the Si film must be increased in order to obtain a crystalline Si having a large grain size. However, since the increase of the film thickness only provides approximately the same crystal grain size as the film thickness, it is principally impossible to prepare a Si film having good crystallinity according to this method. Furthermore, since a high film formation temperature of 600.degree. C. or more is required for this method, a cost problem arises in that it is impossible to use a less expensive glass substrate.
The above-described method (2) requires a heating step at an elevated temperature of 600.degree. C. or more for several tens of hours in the crystallization step. Therefore, this method suffers lower productivity. Since this method utilizes a solid phase crystallization, the resulting crystal grains extend parallel to the surface of the substrate, and some of them even have a grain size of several .mu.m. However, because the grown crystal grains collide each other to form a grain boundary, the grain boundary acts as a trap level for carriers, and may be largely responsible for the reduction of the mobility of the TFT. Moreover, each crystal grain has a twin crystal structure which includes a large amount of crystal defects (which are called a twin crystal defect) even in a single crystal grain.
For these reasons, the above-described method (3) is now mainly used for obtaining crystalline Si semiconductors. Since this method utilizes a fusion solidification method to conduct the crystallization, each crystal grain provides excellent crystallinity. Furthermore, because the selection of the wavelength of the light to be irradiated allows for only the efficient heating of the Si film to be annealed, it is possible to prevent the heat damaging of the glass substrate which is located below the Si film. Moreover, since this method does not require a long-term treatment as used in the method (2), it provides excellent productivity. Since a high power excimer laser annealing device has recently been developed for this method, this method would be applicable to large area substrates.
A method for fabricating semiconductor elements utilizing the above-described method (3) is disclosed in, for example, Japanese Laid-open Patent Publications Nos. 8-201846 and 7-92501.
The method described in Japanese Laid-open Patent Publication No. 8-201846 includes irradiating a driver monolithic-type active matrix substrate for a liquid crystal display apparatus with a pulse laser beam (hereinafter referred to as a laser pulse) in a manner that portions of the laser pulse are overlapped so as to crystallize the Si film corresponding to the active region of the element. The driver monolithic-type active matrix substrate refers to a substrate wherein pixel TFTs and a driver portion which drives the pixel TFTs are simultaneously formed on the same substrate. In addition, this method further includes irradiating the Si film forming the TFT constituting the driver portion with the edge portion of the laser pulse. Furthermore, alternative method is employed which includes irradiating the substrate with the laser pulse in a manner that the width of the semiconductor thin film with respect to the scanning direction of the laser pulse is more than or integral times as much as a pitch of the laser pulse.
This method is the best among methods of crystallizing an Si film on an insulating substrate, but leaves a serious problem in the uniformity of the crystallinity. Specifically, a laser oscillator as a light source having an output power sufficient to irradiate a large area substrate has not yet been developed, and now the surface of the substrate is irradiated by successively scanning a beam having an area of approximately 100-200 mm.sup.2. Therefore, as a matter of course, the non-uniformity of the crystallinity caused by the successive scanning of the laser has become a serious problem. Needless to say, the unevenness of the crystallinity directly relates to the characteristics of the semiconductor element, which causes the unevenness of the characteristics among the elements.
The following illustrates the scanning and irradiation of the laser pulse in more detail. Generally, the scanning and irradiation with the laser pulse is conducted in a manner as illustrated in FIG. 14A. FIG. 14A is a schematic view showing the energy distribution (energy profile) of the laser beam viewed from the cross section in the scanning direction. In FIG. 14A, the symbols 608 and P denote a scanning direction and scanning pitch of the laser pulse, respectively. The energy distribution of each of the laser pulses 601-605 scanned in the scanning pitch P generally provides a Gaussian shape having a beam width 607. The Si film is successively irradiated with the laser pulses in the order of 601, 602, 603, 604 and 605.
Certain points a, b, c and d in the Si film are irradiated a total of three times initially with a laser pulse 602, and subsequently with 603 and 604. That is, the overlapping amount of the laser pulse is set to be about 67% in FIG. 14A. The reason why the laser pulse is scanned and irradiated in a manner that a portion of each of the laser pulses 601-605 is overlapped is to increase the uniformity of the crystallinity of the Si film.
However, the biggest factor determining the crystallinity of the crystalline Si film to be crystallized by irradiation with the laser pulse is an initially irradiated laser pulse. This is due to the following reasons: When an amorphous Si film is crystallized, its melting point is increased from the original melting point by about 200.degree. C. while its absorption coefficient to the laser beam is reduced. On the other hand, the laser pulses which are irradiated at the second time or later do not crystallize the amorphous Si film, but recrystallize the crystalline Si film which has been already crystallized with the first laser pulse. Thus, the effects of the second or later irradiation are greatly reduced, compared to that of the first one. Therefore, the second and later laser pulses do not contribute to the crystallization as the first laser pulse does.
At the locations a, b, c and d in FIG. 14A, the laser pulse 602 is initially irradiated to crystallize an amorphous Si film so as to form a crystalline Si film. Thereafter, the laser pulses 603 and 604 are subsequently irradiated. At the time of irradiating the Si film with the original laser pulse 602, the energy supplied at each of the locations a, b, c and d is shown by the size of the arrow drawn in the vertical direction from each point. The energy is smallest at the location a, while it is largest at the location d. As a result, the crystallinity at the location a will be poorer than that at the location d. Similarly, the crystalinities at the locations b and c are poorer than that at the location d (that is, non-uniformity of the crystallinity occurs depending upon the locations). The laser pulses 603 and 604 are irradiated in order to repair this non-uniformity, however, as described above, these second and later laser pulses do not contribute to the crystallinity as the first laser pulse (602 in this case) does. Therefore, non-uniformity caused by the first laser pulse 602 is not completely repaired at each of the locations a, b, c and d.
The crystallinity distribution of the crystalline Si film thus obtained in the laser scanning direction 608 has a serrated shape as shown by the symbol 609 in FIG. 14B. That is, a periodical non-uniformity is generated due to the laser scanning pitch P, and each of the locations a, b, c and d provides different crystallinity as shown in FIG. 14B. This non-uniformity of the crystallinity is mainly responsible for the non-uniformity of the characteristics of the crystalline Si film which is successively scanned and crystallized with the laser pulses. This causes the unevenness of the element characteristics, which results in display defects such as uneven display (contrast) in, for example, a liquid crystal display device.
Japanese Laid-open Patent Publication No. 8-201846 focuses on the characteristic unevenness in the driver TFT of a driver monolithic-type active matrix substrate for a liquid crystal display device, and suggests a method of reducing the above-described unevenness. This patent publication describes a relationship between the width of the semiconductor thin film and the overlapping amount (i.e., the pitch) of the laser pulse at the time of the successive scanning, wherein the width of the semiconductor thin film is meant to be a width of a separated Si film forming an active region of the TFT (including both a source/drain region and a channel region). Since the TFT characteristics mainly depend on the film quality (crystallinity) of the channel region, it is difficult to accomplish an adequate uniformity intended for a plurality of the driver TFTs even by using the method as described in the above patent publication.
According to Japanese Laid-open Patent Publication No. 7-92501, semiconductor elements (such as TFTs) are disposed on a straight line, and the straight line is irradiated with a laser light while controlling its position so as to crystallize the active region of the semiconductor element. In other words, the laser light is successively irradiated at the precisely controlled position so that the element regions disposed on each straight line are crystallized by a single irradiation with the laser light. Therefore, each TFT is irradiated with a single laser light so as not to create the overlapping portion at the time of the successive scanning. Each element is crystallized using each laser pulse alone and further using a relatively flat region around the peak top portion in its beam profile without creating an overlapping portion as illustrated in FIGS. 14A and 14B. Accordingly, the number of the laser pulses used for crystallizing all the elements on the substrate corresponds to the number of the element lines arranged on the straight lines.
TFT elements obtained by such a method theoretically provide excellent uniformity, but no device available for this technology has been developed yet. This is because the position control in this technology involves problems such as stage precision, the fluctuation of the laser light itself, and the like, and thus actually causes a great difficulty. Even had such a device been developed, the device itself would be very expensive. Additionally, an excessive time would be required for the position control, and the stage transfer speed would be reduced for increasing the position precision. As a result, the reduction of the productivity would cause increased cost.
Another major problem is in a surface roughness resulting from crystallinity. In the crystallization of a silicon film by the fusion solidification using a laser irradiation, the silicon film is instantly heated to a temperature higher than its melting point (1414.degree. C.) and then cooled to room temperature and solidified over a cooling period of several tens of nanoseconds. At this time, the rapid cooling supercools the silicon film, and solidify it instantly. As a result, the resulting crystal grain size is greatly reduced to approximately 100 to 200 nm, and a portion where the crystal grains collide (i.e., crystal boundary) rises to form bump. This phenomenon is more prominent at a point where three crystal grains collide. This phenomenon becomes greater when the crystallinity is better (i.e., when its crystal grain size is larger).
FIG. 15 is a schematic view depicted based on an interatomic force microscope (AFM) image of the surface condition of a crystalline silicon film which is actually crystallized by the strong light irradiation. The full scale in the X-Y direction in FIG. 15 is 2.0 .mu.m, and the full scale in the Z direction is 50 nm. When a capacity component is fabricated using a crystalline silicon film as one of the electrodes, the capacity will be higher than the designed value due to its surface roughness. When its crystallinity is varied by the laser scanning as described with reference to FIGS. 14A and 14B, the resulting silicon film has an increased surface roughness which results in a larger variation of the capacity values. The capacity variation of a storage capacitor connected to the pixel TFT in a liquid crystal display device may cause an uneven display such as flicker of the screen.
As described above, a high performance (for example, high-speed, high-resolution), reliable and stable semiconductor device having less variation of crystallinity among each of the semiconductor elements (for example, TFTs), has been demanded.